Benthic fish, also known as groundfish, are those that stay on or very near the sea bottom, whether in shallow or deep water. Unlike fish that spend their time swimming, benthic fish are very dense and have negative buoyancy, allowing them to effortlessly lie on the bottom or bury themselves. Other adaptations commonly seen in benthic fish include the lack of a swim bladder and a flattened body shape. They are predominantly bottom feeders that eat detritus, or ambush predators that lie in wait for their prey to come within striking distance.
There are many different types of benthic fish. Flatfish, including flounder, halibut, plaice, sole and turbot, lie on the the sea floor or bury themselves in the sand. Asymmetrical physical adaptations include having both eyes on one side of their head, and different pigmentation on each side of their bodies; the side facing down is usually pale, while the side facing up is camouflaged. Some species of flatfish are predators, feeding on smaller fish, while others eat mainly invertebrates.
Rays and skates are also flat, but are bilaterally symmetrical, with their eyes on top of their head. These cartilaginous fish are predators that bury themselves and wait for prey, feeding largely on crustaceans, clams, oysters and snails.
Rattails are benthic fish that have a large head and mouth, and a body that rapidly tapers down to a long, narrow tail. They live in the deep sea, and are scavengers, feeding chiefly on invertebrates. Some species of rattails are brotulas, chimaeras and grenadiers.